There is evidence that a part of the capacity of a host to destroy its own tumor may operate through the macrophage system. It is furthermore thought that the effect of nonspecific stimulants such as BCG and C.parvum may be through stimulation of the macrophage system. The tumor immunology program, NCI, wishes to support studies concerning the role of macrophages in the tumor specific and general immune response of tumor bearing hosts. These studies should lead towards a definition of any macrophage activities that cause tumor destruction and also should attempt to define and measure any defects in macrophage function that might exist in tumor bearing hosts. Studies of the effects produced by anti-tumor therapy (e.g., drugs, X-ray) on macrophage function may also be included. Tests of macrophages may include in vivo and in vitro studies of their function, quantity, distribution, and other properties.